Analog-to-digital converters are used in applications where a digital representation of an analog value is needed. Two of the primary criteria of A/D converters are the resolution or number of digital bits in the output, and the speed required for each conversion. One method of achieving very high resolution A/D converters is by using a precision, multiple-winding transformer as a D/A converter in a successive-approximation A/D conversion. One such transformer-type A/D converter is described in U.S. Pat. No. 3,926,056 to Neil L. Brown for "Conductivity, Temperature and Pressure Measuring System."
The accuracy of a transformer A/D converter may be made very high. Precision ratio transformers can be designed which are capable of maintaining output voltage ratios precisely proportional to turns ratios. In special cases, the precision of the voltage ratios can be as high as one part in 10.sup.8, and the accuracies which can be achieved by successive approximation A/D converters using this type of transformer are comparable. Transformer A/D converters are especially suited to applications in which the transducer whose output is being digitized requires an AC excitation signal.
Although the use of a precision ratio transformer allows extremely accurate analog-to-digital conversion, the use of such a transformer with a successive approximation technique has certain disadvantages. While the digital representation provided by a successive approximation technique in response to a varying input signal is an accurate representation of the instantaneous value of that signal at some point in time during the conversion interval, there is no indication of exactly when during the conversion interval this point occurred. Furthermore, when compared with either the average value of the input signal over the entire conversion interval or the value of the signal at any predetermined point during the conversion interval, the digital representation of an increasing input signal has a positive bias while the representation of a decreasing input signal has a negative bias. In other words, the bias or round-off error in the digital representation is correlated with the slope of the signal. This error characteristic is unacceptable for many types of applications in which large numbers of digital values are statistically reduced to provide meaningful data. While the addition of circuits such as sample-and-hold input stages can eliminate this correlation, the design of these circuits with sufficient accuracy for very high resolution A/D conversions--16 to 20 bits and higher--is expensive and difficult.